1. Field of the Invention
The present invention relates to digital imaging apparatus and methods, and more particularly to a system for imaging recording constructions (such as lithographic printing members) using digitally controlled laser output.
2. Description of the Related Art
Numerous industrial and graphic-arts applications require pinpoint delivery of laser radiation to the surface of a rotating drum. Such applications include, for example, copying, printing and proofing applications in which the radiation is used to expose a recording member. By rotating the drum while drawing the laser source in an axial direction, a complete scan of the recording construction is achieved. During the course of the scan, the laser source is activated in an imagewise fashion to produce a series of image dots at appropriate locations on the recording member. Depending on the system, the image dots may alter the recording member directly or in a latent sense, requiring subsequent development.
For example, U.S. Pat. Nos. 5,351,617 and 5,385,092 disclose ablative recording systems that use low-power laser discharges to remove, in an imagewise pattern, one or more layers of a lithographic printing blank, thereby creating a ready-to-ink printing member without the need for photographic development. In accordance with those systems, laser output is guided from a laser diode to the printing surface and focused onto that surface (or, desirably, onto the layer most susceptible to laser ablation, which will generally lie beneath the surface layer). Other systems use laser energy to cause transfer of material from a donor to an acceptor sheet, to record non-ablatively, or as a pointwise alternative to overall exposure through a photomask or negative.
As discussed in the '617 and '092 patents, laser output can be generated remotely and brought to the recording blank by means of optical fibers and focusing lens assemblies. Commercial systems typically employ an array (usually, but not necessarily linear) of lasers and associated focusing assemblies in order to reduce overall imaging time. Each assembly images over a circumferential band, the width of which defines the total axial movement of the array during the course of scanning.
A representative system is illustrated in FIGS. 1A and 1B. The system includes a cylinder 100 around which is wrapped a lithographic plate blank 102. Cylinder 100 includes a void segment 105, within which the outside margins of plate 102 are secured by conventional clamping means (not shown). Cylinder 100 is supported in a frame and rotated by a standard electric motor or other conventional means. The angular position of cylinder 100 is monitored by a shaft encoder 108. A writing array 110, mounted for movement on a lead screw 112 and a guide bar 115 (see FIG. 1B), traverses plate 102 as it rotates. Axial movement of writing array 110 results from rotation of a stepper motor 118, which turns lead screw 112 and thereby shifts the axial position of writing array 110. Stepper motor 118 is activated during the time writing array 110 is positioned over void 105, i.e., after writing array 110 has passed over the entire surface of plate 102. The rotation of stepper motor 118 shifts writing array 110 to the appropriate axial location to begin the next imaging pass.
The axial index distance between successive imaging passes is determined by the number of imaging elements in writing array 110 and their configuration therein, as well as by the desired resolution. The imaging elements may be a series of independently addressable diode lasers whose outputs are conducted to associated focusing assemblies 120. These are evenly distributed along the linear writing array 110. The interior of writing array 110, or some portion thereof, contains threads that engage lead screw 112, rotation of which advances writing array 110 along plate 102 as discussed previously.
With reference to FIG. 2, the imaging radiation that strikes plate 102 originates with a series of laser sources, one of which is representatively indicated at 200. The output of laser 200 is guided, by means of a fiber-optic cable 205, to an associated focusing assembly 120. Laser 200 is selectably switched on and off by one of a series of laser drivers 210. A controller 215 operates laser drivers 210 to produce imaging bursts when the various focusing assemblies 120 reach appropriate points opposite plate 102.
Controller 215 receives data from two sources. The angular position of cylinder 100 with respect to the laser output is constantly monitored by shaft encoder 108, which provides signals indicative of that position to controller 215. In addition, an image data source (e.g., a computer) 220 also provides data signals to controller 215. The image data define points on the plate 102 where image spots are to be written. Controller 215, therefore, correlates the instantaneous relative positions of focusing assemblies 120 and plate 102 (as reported by encoder 108) with the image data to actuate the appropriate laser drivers at the appropriate times during scan of plate 102.
Assembly 120 contains a focusing lens that focuses radiation from cable 205 onto the surface of plate 102, concentrating the entire laser output onto plate 102 as a small feature to achieve high effective energy densities. The distance S between the output lens of focusing assembly 120 and the surface of plate 102 is chosen so that the beam is precisely focused on the surface, as indicated in FIG. 2.
Actually conforming all of the focusing assemblies 120 to this ideal, however, is very difficult as a realistic matter. Even slight differences among assemblies 120 in terms of the distance S can affect imaging performance. This is because any deviation from perfect focus results in lost energy density, since by definition the point of perfect focus is where power is most highly concentrated. Slight skew or yaw of the writing head 110 with respect cylinder 100, or differences in the mounting configurations for the focusing assemblies within writing head 110, can result in different effective laser energy densities reaching the plate 102. In printing applications, this translates into different exposure densities at the plate, and consequent printing-density variations on the final copy.